Methods of X-aptamer generation and compositions thereof

ABSTRACT

Provided herein are methods for a novel bead-based next-generation “X-aptamer” selection scheme that extends aptamer technology to include X-modified bases, thus resulting in X-aptamers, at any position along the sequence because the aptamers are chemically synthesized via a split-pool scheme on individual beads. Also provides are application to a wide range of commonly used DNA modifications, including, but not limited to, monothioate and dithioate backbone substitutions. This new class of aptamer allows chemical modifications introduced to any of the bases in the aptamer sequence as well as the phosphate backbones and can be extended to other carbohydrate-based systems.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority based on U.S. Provisional ApplicationSer. No. 61/711,915 filed Oct. 10, 2012, which is incorporated herein byreference.

STATEMENT REGARDING GOVERNMENT INTERESTS

This invention was made with Government support under HHSN272200800048C,HHSN268201000037C, CA151668, HD080020, AI054827, 275200800020C, andGM092599 awarded by The National Institutes of Health and underW81XWH-09-1-0212 and W81XWH-09-2-0139 awarded by The Department ofDefense. The Government has certain rights in the invention.

FIELD OF THE INVENTION

This invention relates generally to methods for selection of aptamershaving small molecule substituents. The present invention relates moreparticularly to combining random or pseudo-random bead-based aptamerlibraries with conjugation chemistry to produce modified aptamers havingenhanced chemical functionality.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is describedin connection with aptamers generally and traditional aptamer generationmethodologies.

Aptamers are structurally distinct RNA and DNA oligonucleotides (ODNs)that can mimic protein-binding molecules and exhibit high (nM) bindingaffinity based on their unique secondary three-dimensional structureconformations and not by pair-wise nucleic acid binding. Aptamers can beselected via high-throughput in vitro methods to bind target molecules.Aptamers are thus emerging as viable alternatives to small molecules andantibody-based therapies in the field of drug development.

Aptamers are typically approximately 1/10th the molecular weight ofantibodies and yet provide complex tertiary, folded structures withsufficient recognition surface areas to rival antibodies. However,aptamers achieve their selectivity through a very limited repertoire offunctional groups—the sugar phosphate backbone and 4 bases. In contrastantibodies use all 20 amino acids with a full range of chemicalsubstituents including positively-charged, sulfhydryl, hydrophobicsidechains, etc. Aptamers are polyanions, potentially limiting theiraffinity towards the full diversity of proteins. It is often difficultto select an aptamer targeted to very acidic proteins because there areno cationic groups to neutralize anionic surfaces on the protein. Whileoligonucleotide agents show therapeutic promise, various pharmacologicalproblems must be overcome. High sensitivity to nuclease digestion makesoligonucleotide agents unstable and thus impracticable for in vivoadministration by either intravenous or oral routes.

In fact, a diverse range of modifications at all possible modificationsites of an oligonucleotide have been reported for enhancingoligonucleotide drug properties including in vivo stability. Theseinclude alterations of linkages (backbones), heterocycles,carbohydrates, and connection and conjugation sites, as well as thecomplete removal of the sugar-phosphate backbone. See, e.g. Yang X; LiN; Gorenstein D G Expert Opin Drug Discov 6 (2011) 75-87; Brody E N, etal. Expert Rev Mol Diagn 10 (2010) 1013-22; Keefe A D, Cload S T CurrentOpinion in Chemical Biology 12 (2008) 448-456.

Certain of the present inventors have developed thiophosphate-backbonemodified aptamers (“thio” aptamers) as specific protein-binding reagentsthat are endowed with nuclease resistance. See, e.g. Yang, X. et al.Nucleic Acids Research 30 (2002) e132; Yang, X. et al. Nucleic AcidsResearch 31 (2003) e54. Oligonucleotides with high monothio- ordithiophosphate backbone substitutions enhance the specificity andaffinity of these agents for the desired protein target and also enhancethe nuclease stability.

However effort to combine the best attributes of antibodies, smallmolecules and aptamers has remained elusive. Selection of aptamers bythe classical iterative selection-amplification method followed bypost-selection modification has been disappointing because themodifications affect the three dimensional structure of the aptamer,which is the basis of its ability to bind to the target by which it wasselected. It has been shown that certain substituents can be introducedinto the bases of the oligonucleotides to provide additionalfunctionalities. For instance, the 5-position of dU can be replaced witha range of substituents (X) and still allow Taq and other polymerases toamplify the selected sequences. Thus, with the appropriate 5-X-dUTP, itis possible to amplify a selected sequence during the in vitro iterativeSELEX scheme and create a large initial random library (10¹⁴ differentsequences), then select a subset that binds to the target protein,amplify and repeat this cycle—often 10-15 cycles are required. Theproblem is that every 5-X-dU position will have the same modifiedX-substituent.

From the foregoing it is apparent there is a need in the art for robustmethods that allow the selection of modified aptamers having desiredchemical substituents. The invention described herein provides novelmethods for achieving this end.

BRIEF SUMMARY OF THE INVENTION

Disclosed herein are methods for a novel bead-based next-generation“X-aptamer” selection scheme that extends aptamer technology to includeX-modified bases (thus resulting in X-aptamers) at any position alongthe sequence because the aptamers are chemically synthesized via asplit-pool scheme on individual beads. As used herein, the termX-aptamer refers to a subclass of aptamers that have as a part of theirsequence chemically modified oligonucleotides that are not the substrateof a polymerase enzyme. As such, X-aptamers cannot be generated by aniterative SELEX type selection but must be selected from one-beadlibraries generated by split and pool synthesis. This new technology iscompatible with a wide range of commonly used DNA modifications,including, but not limited to, monothioate and dithioate backbonesubstitutions. This is a new class of aptamer, where any chemicalmodifications can be introduced to any of the bases as well as thephosphate backbones and could be extended to other carbohydrate-basedsystems.

In one embodiment a system is provided for identifying both novelX-aptamer affinity agents that feature small molecule X-groupmodifications on the bases and may (or may not) contain partially orfully modified phosphate backbones using a bead-based combinatoriallibrary of X-aptamers as well as one or more target molecules thatinteract with a specific X-aptamer. The system includes use of apartially or fully-modified oligonucleotide-bead X-aptamer combinatoriallibrary with small molecule X-group modifications on the bases forbinding one or more target molecules, wherein each bead of the X-aptameroligonucleotide-bead library comprises more than one copy of a uniqueoligonucleotide having a unique sequence and backbone and basemodification(s). Target molecules may include proteins, peptides,carbohydrates, small molecules, intact cells, virions, etc. In certainembodiments, the modified backbones include monothioate and dithioatebackbone substitutions.

In one embodiment a method for isolating a target specific X-aptamer isprovided that includes the steps of generating an oligonucleotidelibrary containing a plurality of different oligonucleotide sequencesusing bead based split and pool synthesis wherein at least one chemicallinker derivitized nucleotide is pseudo-randomly inserted into theoligonucleotide sequences and wherein the chemical linker provides forattachment of an X-ligand into the oligonucleotide sequences therebyforming an X-aptamer library, and identifying a target specificX-aptamer sequence by target binding and sequence determination. Incertain embodiments the oligonucleotide library is partiallythio-modified or fully thio-modified. The unmodified oligonucleotideversion of the target specific X-aptamer can be determined by nucleicacid sequencing or can be determined by polymerase chain reaction (PCR)amplification. The sequence serves as a sequence barcode for determininga step of the split and pool synthesis at which a chemically modifiedbase was inserted.

The X-ligands can be attached to the chemical linker prior to creationof the oligonucleotide library, between split and pool steps duringcreation of the oligonucleotide library or after generation of theX-aptamer library. The X-ligands can be pre-coupled to one or morephosphoramidites used to synthesize the oligonucleotide library.

In certain embodiments a combinatorial X-aptamer oligonucleotide libraryon bead supports is generated from which target specific X-aptamers areisolated by target binding. The library can be generated by a processincluding the steps of:

-   -   a) establishing a reaction column for each subunit        oligonucleotide base species that will be used to synthesize a        random collection of oligonucleotide sequences with modified        base and/or backbones and distributing a set of activated or        prederivitized supports in each reaction column;    -   b) attaching a single nucleotide species to the supports in each        of the columns wherein the single nucleotide species comprises a        backbone substituted with a protected normal phosphate ester or        a modified phosphate ester that is not a substrate for a        polymerase enzyme and/or wherein a base of the single nucleotide        species is unmodified or modified with a chemical linker that        provides for attachment of an X-ligand to the oligonucleotide;    -   c) mixing the supports from the columns together;    -   d) splitting the mixed supports back into each of the columns;        and    -   e) repeating steps b)-d) until an oligonucleotide of a desired        length is obtained, wherein each support comprises many copies        of a unique X-aptamer oligonucleotide sequence, wherein each        unique sequence contains at least one nucleotide species        including a modified phosphate ester that is not a substrate for        a polymerase enzyme and/or at least one nucleotide species that        is modified with a chemical group X.

The aforementioned library can be generated where the X-ligands havebeen previously chosen to bind to a target molecule by in silicomolecular modeling and the rest of the X-aptamer oligonucleotide servesas a scaffold to present one or more small-molecule X-ligands to bind tothe target molecule. In certain embodiments, the subunit oligonucleotidebase species are phosphoramidites or thiophosphoramidites and theX-ligands have been previously attached to the precursorphosphoramidites or thiophosphoramidites. The X-ligands may be attachedby chemical process known to those of skill in the art including by“click chemistry” or amide coupling chemistry.

In certain embodiments, the process of adding the X-ligand begin with apredetermined or established aptamer or X-aptamer sequence.

Target specific X-aptamers are isolated from the combinatorial X-aptameroligonucleotide library by selection of X-aptamers that specificallyinteract with a desired target. The sequences of the target specificX-aptamers are obtained. In one embodiment a nucleic acid sequencer isused to determine the sequence of the modified oligonucleotide withrandom X-group modifications on the bases (X-aptamer). Alternatively,oligonucleotides with random X-group modifications on the bases are PCRamplified to generate an unmodified version of the oligonucleotide andeither sequenced directly or cloned prior to sequencing. Modification ofthe base with the X-group will still allow PCR amplification, albeitwith an unmodified sequence that then serves as a barcode for the beadand later allows identification of the X-modifications and backbonemodifications. The X-groups may have been previously chosen by any of anumber of methods, including without limitation, groups predicted tobind to the target molecule by in silico molecular modeling, groupsidentified by high-throughput chemical screens, and/or using SAR by NMR(“structure-activity-relationships” by NMR) studies of small moleculeslibraries. The X-aptamer oligonucleotide serves as a scaffold to presentone or more small-molecule X-groups to bind to the target molecule.

In certain embodiments, a protein separator is included that separates abound protein into fragments prior to separation by liquidchromatography followed by mass spectrometry. The one or more proteinsbound to the oligonucleotide X-aptamer may be extracted and separatedprior to identifying the proteins using liquid chromatography, massspectrometry (MS), liquid chromatography, and/or time of flight (TOF)mass spectrometry and combinations thereof.

In certain embodiments the oligonucleotides are attached to the supportsusing a chemical linker that is not cleaved upon oligonucleotidedeprotection. Heat- or light-sensitive linkers attaching the X-aptamerto the bead may also be used.

In certain embodiments of the bead based library, the X-groups have beenpreviously attached to the precursor phosphoramidites orthiophosphoramidites or the X-groups are attached between split and poolsteps by “click chemistry” or amide-carboxyl coupling chemistry or bythese and other coupling steps at the completion of the random librarycreation using bases incorporating such groups as azide or ethynylX-groups (allowing for click chemistry), or carboxyl or amine groups(allowing for amide coupling chemistry).

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, includingfeatures and advantages, reference is now made to the detaileddescription of the invention along with the accompanying figures:

FIG. 1A shows the sequences of preselected aptamers. FIG. 1B shows thesequences programmed into the synthesizer. CL1-CL4 represents thesynthesis sequences programmed into the four-column synthesizer. Pooland split steps are indicated by asterisks. The resulting libraryconsists of 1,048,576 (4¹⁰) possible sequences. An example of onepossible X-aptamer sequence from the library is shown, representing abead that starts (from the 3′ end) in column 3, and then is transferredin successive pool and split steps (3′ to 5′) to columns 4, 2, 4, 1, 3,2, 4, 2, 1 (underlined sequence blocks). FIG. 1C shows the sequences ofobtained X-aptamers.

FIG. 2A shows a set of NMR spectra measured for CD44-HABD in thepresence of different concentrations of the ADDA ligand titration ofCD44-HABD. FIG. 2B shows the intensity changes of peaks that wereselected in order to obtain a statistical value of the dissociationconstant of the ligand ADDA.

FIGS. 3A and B respectively illustrate the binding curves of ADDA XA3(SEQ ID 14, where X=ADDA-modified dU) and ADDA XA7 (SEQ ID 18, whereX=ADDA-modified dU).

FIGS. 4A-C illustrate the predicted secondary structures of selected XAsequences.

FIGS. 5A-C respectively represent the binding curves of truncated Motifs3, 4 and 5.

FIG. 6 shows flow cytometry results of ovarian cancer cell line IGROV(CD44+) incubated with Motif 3 (amino form and ADDA adduct) at 37° C.for 2 hours.

FIG. 7A shows the structure of a 7-mer hyaluronic acid (HA), FIG. 7Bshows structure of the ADDA ligand, and FIG. 7C shows a docking overlayof HA (bond representation) with ADDA ligand (circled) on the CD44-HABD.

FIG. 8 shows the reaction of azides and alkynes to form triazoles in thepresence of copper.

FIG. 9 shows examples of building blocks for azide click chemistry.

FIG. 10 shows an example of a click chemistry alkyne reaction.

FIG. 11 shows the results of a time wise progression of click chemistryby HPLC (A), mass spectroscopy (B), and gel electrophoresis (C).

FIG. 12 depicts the structures of a normal phosphate compared withphosphomonothioate and phosphodithioate.

FIGS. 13A-D depict structures of certain exemplary base modificationsfor use in XA.

FIG. 14 is a cartoon depicting three split and pool steps for bead basedlibrary generation

FIG. 15 is a cartoon of the general isolation method for target specificbeads from a bead library.

FIG. 16 shows the results of Near Infrared (NR) images of an IRdye800-labelled ESTA-1 thioaptamer—gold nanoparticle binding in a mousemodel of human pancreatic cancer.

FIG. 17 shows the two dimensional stem-loop structure of the ESTA-1thioaptamer.

FIG. 18 shows one embodiment of a pathway for generating a drug bearingX-aptamer.

FIG. 19 presents data on Mean Fluorescence Intensity (MFI) of IGROVcells incubated with X-aptamer motifs at 37° C. for 2 hours.

FIG. 20 presents data on Mean Fluorescence Intensity (MFI) of IGROVcells incubated with dithioated X-aptamer motifs at 37° C. for 2 hours.

FIG. 21A presents the sequences of X-aptamers isolated for CD44 bindingwith a pteroic acid moiety attached.

FIG. 21B structure of pteroic acid.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts whichcan be employed in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention and do not delimit the scope of theinvention.

ABBREVIATIONS: The following abbreviations are used throughout thisapplication:

-   -   5-NH₂-dU 5-(aminoethyl-3-acrylimido)-deoxyuridine    -   ADDA N-acetyl-2,3-dehydro-2-deoxyneuraminic acid amino-dU        5-(aminoethyl-3-acrylimido)-deoxyuridine    -   CD44-HABD hyaluronic acid binding domain of CD44    -   ODN Oligodeoxynucleotide    -   PCR Polymerase Chain Reaction    -   SAR by NMR Structure-Activity-Relationships determined by        Nuclear Magnetic Resonance (NMR)    -   SELEX Systematic Evolution of Ligands by Exponential Enrichment    -   TA Thio-aptamer    -   TBTA tris(benzyltriazolylmethyl)amine    -   XA X-aptamer

To facilitate the understanding of this invention, a number of terms aredefined below. Terms defined herein have meanings as commonly understoodby a person of ordinary skill in the areas relevant to the presentinvention. Terms such as “a”, “an” and “the” are not intended to referto only a singular entity, but include the general class of which aspecific example may be used for illustration. The terminology herein isused to describe specific embodiments of the invention, but their usagedoes not delimit the invention, except as outlined in the claims.

For purposes of the present invention, the acronym “SELEX” refers to theiterative selection and amplification aptamer selection method describedin 1990 by Tuerk and Gold (Science 249 (1990) 505-510) and Ellington andSzostak (Nature 346 (1990) 818-822). As originally described, SELEXbegins with a library of soluble oligonucleotides that is contacted withtarget compounds followed by partitioning of those nucleic acids havingan increased affinity to the target from the candidate mixture. Thepartitioned nucleic acids are amplified by PCR and, in an iterativeseries of selection and amplification steps, enrichment and isolation ofspecific high affinity aptamers is obtained. See, e.g. Gold and Tuerk,U.S. Pat. No. 5,270,163, describing an in vitro combinatorial method forthe identification of nucleic acid ligands.

Combinatorial chemistry involves linking together, in an essentiallystep-wise fashion, identical or non-identical building blocks such asmonomeric subunits, chemical groups, and the like, to form libraries ofnew compounds. The term “library” as used herein refers to a collectionof different individual molecules that have a common generic structureand are produced by combinatorial chemistry. Preferably, the library isdesigned to contain significant if not nearly equal representation ofall possible different individual molecules that can be theoreticallygenerated given the chemistry and added constituents. The term “library”in the context of the present invention also refers to the products ofsplit combinatorial synthesis of organic molecules having a common corestructure or template which has a discrete number of independentlyvariable substituents, each of which can have one of a defined range ofvalues. Such templates may have a number of different functional sites,including those where each site is amenable to a different couplingchemistry and where a plurality of different substituents are introducedfor binding to a different site at succeeding coupling steps.

Aptamers constitute one class of oligonucleotide molecules derived fromcombinatorial chemistry. Aptamers are oligonucleotides (double or singlestranded DNA or RNA molecules) that fold into sequence dependent threedimensional structures and are biologically active on the basis ofresulting structure based interactions (such as decoys) or catalyticproperties (such as antisense, ribozymes or siRNA). Identifying usefulaptamers, or oligonucleotides having biologic activity on the basis oftertiary structure, requires the generation of large candidate librariesof random sequence or backbone modification oligonucleotides as well asselection, identification and reproduction of the rare structures thatare able to interact with a given template. Identification of aptamerstructures in libraries of oligonucleotides having regions of definedsequence as well as randomized sequence and backbone modifications suchas thioaptamers can be performed by in vitro selection and amplificationby PCR.

The bead-based split synthesis selection process disclosed and utilizedherein is distinct from the above referenced SELEX methodology. Splitsynthesis as originally adapted to generation of single bead peptidelibraries was developed by certain of the present inventors to generateone-bead one-oligonucleotide libraries where each bead presents manycopies of a single oligonucleotide sequence or species. See, e.g. YangX, et al. Construction and selection of bead bound combinatorialoligonucleoside phosphorothioate and phosphorodithioate aptamerlibraries designed for rapid PCR-based sequencing. Nucleic AcidsResearch 30 (2002) e123; Yang X, et al. Immunofluorescence assay andflow-cytometry selection of bead-bound aptamers. Nucleic Acids Research31(10) (2003) e54; Gorenstein D G, et al. “Bead Bound CombinatorialOligonucleoside Phosphorothioate and Phosphorodithioate AptamerLibraries” (U.S. Pat. No. 7,338,762).

In one embodiment, copies of a single, chemically pure phosphorothioateoligonucleotide (S-ODN) are introduced onto each bead by the ‘mix andseparate’ split synthesis method. Although oligonucleotides arerelatively chemically stable, they are particularly susceptible toenzymatic degradation by nucleases. Controlled inclusion of modifiedresidues such as thiophosphate (S-ODN) and dithiophosphate (S₂-ODN)residues is able to confer nuclease resistance and improve the bindingproperties of aptamers. See Gorenstein et al., U.S. Pat. No. 6,423,493.

In one embodiment, polystyrene beads with a non-cleavablehexaethyleneglycol linker attaching the first phosphoramidite are usedsuch that the synthesized ODNs are still covalently attached to thebeads after full base and phosphate ester deprotection. The aptameroligonucleotide chains described herein will typically have sectionsthat are non-random. For example, in a typical aptamer, at least the 5′and 3′ termini constitute preselected sequences of PCR primers and maybe generated by first nonrandom programmed stepwise addition to supportsin one or more of the synthesis chambers. The 5′ and 3′ primer sequencesmay have functional roles in the ultimate aptamer. For example, the 5′and 3′ sequences may be designed to contribute to a resulting stem-loopstructure as will be later discussed.

The bead based process avoids the many rounds of solution enrichment andamplification of potential binding agents required by SELEX, and so canbe accomplished much faster than SELEX, usually in one or two rounds.This is because each bead of the bead based library contains thousandsof copies of the identical sequence and will therefore capturesufficient labeled target to be selectable in the first instance. WithSELEX there will not be detectable numbers of copies of a given sequencefor many rounds of amplification. Additionally, while the SELEX processis limited to binding agents (aptamers) consisting of nucleic acids thatcan be generated enzymatically, the bead-based process is notconstrained by the type of nucleic acids (normal or chemically modified)used in the starting library.

Where identification of the target selected oligonucleotides is to beconducted by PCR, the only limitation on applicable chemicalmodifications is whether a chemically modified sequence can be read bythe DNA polymerase used in PCR. The location of the modification isdetermined by comparing the selected sequence with the column program todetermine where the modification must be. With the sequence and themodification site in hand, the identified X-aptamer can be synthesized.In contrast, in SELEX, the PCR product must be a faithful copy of theoriginal sequence which is impossible for many modifications because theDNA polymerase will only copy the sequence using unmodifiednucleotides—it is unable to build a faithful copy that includes themodifications for the further required iterative rounds.

For purposes of the present method using PCR to read the selectedsequence, all possible types of DNA modifications that can be chemicallysynthesized can be used with this method so long as a nucleic acidpolymerase can read the sequence. Examples of potential modificationsinclude 5′-dyes, 5′-chemical linkers, and 5′-metal chelators. Certain ofthese may be useful for visualization of beads binding to tissue, forexample.

Also suitable for use with PCR reading of the selected sequence are anynucleic acid polymerase readable sugar polymer types, such as forexample arabinose-based polymers. If readable by PCR, where this methodof determining the sequence is employed, modifications can be either inthe backbone, the deoxyribose (and ribose) sugars, or the bases.

Non-limiting examples of backbone modifications include phosphate(normal), monothioate, dithioate, methyl phosphonate, alkyl phosphonate.FIG. 12 shows the structures of monothioate and dithioate phosphatescompared with a normal phosphate. The thioates provide enhanced nucleasestability and can enhance aptamer binding affinity without sacrificingspecificity. As can be seen, the phosphorodithioates are achiral atphosphorus. The thioates can be read in a sequence bearing them by PCRand, after identification of the thiolated bases by reference to thecolumn protocol used to generate the library, thioates can beincorporated at selected positions during chemical synthesis aftertarget binding. The dithioates are incompatible with conventional SELEXbecause they cannot be incorporated into synthesized stands by thepolymerase.

Non-limiting examples of sugar modifications include(Deoxy)-Ribose-2′-fluoro, 2′-OMe, 2′-methyl, and2′-deoxy-2′-fluoro-D-arabinose.

Non-limiting examples of base substitutes include5-(3-aminoallyl)-deoxyuridyl, 5-(alkynyl)-deoxyuridyl, and3-(2-Deoxy-b-D-ribofuranosyl)-1,3-diaza-2-oxophenothiazine. Many othermodifications of base substitutes are possible so long as a nucleic acidpolymerase can read the sequence that includes the modification. FIGS.13A-D shows the structures of several exemplary base substitutesincluding: 5-(3-aminoallyl)-deoxyuridyl (FIG. 13A); 5-(3-aminoallylleucyl)-deoxyuridyl (FIG. 13B); 5-(3-aminoallylphenylalanyl)-deoxyuridyl (FIG. 13C); and5′-Dimethoxytrityl-5-(pyren-1-yl-ethynyl)-2′-deoxyUridine,3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite (FIG. 13D). Thevarious potential base substitutions permit virtually unlimited chemicalfunctionality including addition of positive charges, hydrophobicgroups, amino acids, and small molecule drugs. After selection andidentification, the base substitutions can be easily incorporated atselected positions, directly during synthesis or post-syntheticallyusing amide coupling or click-chemistry. The present technology providesa means to include these modifications, which are incompatible withtechniques such as SELEX that rely on amplification of faithful copieswith each round of selection.

In one exemplified embodiment of the present disclosure, high bindingaffinity partially monothioate DNA aptamers are first selected against adesired target. These aptamers can be selected by methods such as SELEXor from bead-based libraries as generally depicted in FIG. 15. It isnoted in this context and for purposes of clarification, that SELEX canonly be used to select partially monothioate aptamers as used as thestarting material of EXAMPLE 1. For fully monothioate aptamers asstarting materials, a bead-based process would be employed because SELEXcannot be used to prepare fully monothioate aptamers.

As shown on FIG. 15, in bead based library selection each bead isconstructed to have many copies of the same unique sequence on itssurface. After binding to labeled target, beads binding high amounts ofthe target are selected and isolated from the remaining majority ofbeads, which bind no or low amounts of the target. Bead selection can beachieved by any suitable method. For example, the target can be renderedfluorescent (by attachment of fluorescent dyes), and beads that bindlarge amounts of the target can be identified by their high fluorescencerelative to other beads. Such beads can be isolated by manual recoveryusing a micropipettor, by automated fluorescence-activated sorting. Thesequences on the selected beads are determined, most typically by PCRcombined with sequencing and characterization of the sequence. WhereX-groups have been added to the sequences during construction, thelocation of the X-groups is determined by consulting the program bywhich the nucleotides were added to the beads. This method hasconsiderable advantages including very high selective enrichment,isolation in a single cycle, no PCR amplification bias and no chemistrylimitations, except, in the case of sequence determination by PCR, thatthe nucleic acid polymerase be able to read the sequence on the bead.

FIG. 14 presents a simplified 2 column depiction of the output of thebead library synthesis process through three split and pool steps. In apreferred embodiment, the synthesis is undertaken in a fully automatedDNA split pool synthesizer, which was developed by certain of thepresent inventors. Through this method, chemical modifications can beincorporated randomly into the library at any position, not justconventional dNTPs.

In the examples presented herein, pre-selected aptamers served as thelead sequences for the design of high-sequence-diversity XA although itis also possible to incorporate X groups into a random library from thebeginning That is, it's not necessary to start with an existing aptamersequence and then try to improve it by adding X groups at randompositions.

One embodiment presented herein provides the first example of X-aptamersthat are endowed with both nuclease resistance and expanded chemicalfunctionalities, specifically drug-like molecules added to 5-positionsof certain uridines on a completely monothiophosphate-backbonesubstituted oligonucleotide aptamer. By combining one-bead, one-sequencethioaptamer selection method with the incorporation of pseudo-randomlyplaced bases containing chemical linkers, additional X-ligands can beappended onto aptamers or thioaptamers to create a next-generation,X-aptamer library, and the best binding X-aptamers can be selected fromthis large pool of sequences.

In one embodiment disclosed herein, an exemplary reaction incorporatesan X- moiety, 5-[N-(2-aminoethyl)-3-(E)-acrylamido]-2′-deoxyuridine,referred to herein as “amino-dU” to various positions in the XA libraryvia Glen Research (22825 Davis Drive, Sterling, Va., 20164, US) AminoModifier C2 dT phosphoramidite.

The structure of amino-dU is shown below:

The structure of amino C2 dT is shown below:

Four sequences were programmed for four different columns and designedfor coding the positions of amino-dU. This library served as a basisfrom which additional X-aptamer libraries were made by conjugatingthrough amide chemistries different carboxylated chemicals (ligands,drug-leads) onto the amino groups. As will be later described, this canalso be achieved by using “click” chemistry of alkyne and small moleculeazides. The amino-dU modification exemplified in Example 1 herein isjust one of a large number of modified bases that can be used in such anapproach. The bead-based selection and barcoding identificationprocesses can accommodate any modified 5-X-dU phosphoramidite (even alarge hydrophobic pyrenyl group). Due to the barcoding system, a T or Xcan be put into any position so long as the sequence is readable by PCRif PCR is to be used for sequence determination.

The X-groups may have been previously chosen by any of a number ofmethods, including without limitation, groups predicted to bind to thetarget molecule by in silico molecular modeling, groups identified byhigh-throughput chemical screens, and/or using SAR by NMR studies ofsmall molecules libraries (Shuker et al. Discovering High-AffinityLigands for Proteins: SAR by NMR. 274 Science (1996) 1531-1534).

In contrast, while some modified bases can be accommodated in thetraditional SELEX methodology, many cannot because the modificationitself must be incorporated into the ODN during PCR in order to beselected through SELEX. In contrast, in the processes disclosed herein,it is only required that the modified template can be read out by thepolymerase. Multiple different modifications can be incorporated ateither random or pre-specified positions within a single library. Thesecapabilities dramatically expand the binding potential andfunctionalities of thioaptamers/X-aptamers.

The present disclosure provides a significant improvement to thegeneration of representational combinatorial libraries by automatedsynthesis in that the resulting X-aptamers feature both nucleaseresistance and expanded chemical functionalities. The realized goal ofthe research leading to the present disclosure was to develop a newclass of X-aptamers with enhanced binding affinity and specificity to atarget protein. By introducing protein binding small drug molecules intothe 5-position of dU residues at the random position of the aptamersand/or replacing one or more of the non-bridging phosphate oxygens withsulfurs, we are able to select an X-aptamer with 40 nM affinity to theexemplary CD44-HABD molecule through a non-iterative bead-basedselection from large combinatorial libraries. Unlike the originalaptamer concept, which mainly depends on the scaffold and sequence ofthe self-folding oligonucleotides, the X-aptamer has simultaneousinteractions with target protein resulting from conjugated organicbinding partners (or even amino-acid-like sidechains) as well as theX-aptamer oligonucleotide backbone. Such X-aptamers have applicabilityto the regulation of cancer related proteins, delivery ofcancer-fighting drugs directly to cancer cells, and imaging anddiagnostics among others.

The following examples are included for the sake of completeness ofdisclosure and to illustrate the methods of making the compositions andcomposites of the present invention as well as to present certaincharacteristics of the compositions. In no way are these examplesintended to limit the scope or teaching of this disclosure.

EXAMPLE 1 Complete Monothioate Backbone Substituted Aptamer to CD44-HABD

As one example of starting material, in a previous report, certain ofthe present inventors described thioaptamers substituted withmonothiophosphates on the 5′ side of dA that bind to the hyaluronic acidbinding domain of CD44 (CD44-HABD) (K_(D)=187-295 nM). Somasunderam A etal. Biochemistry 49 (2010) 9106-9112. The sequences of the originalpreselected aptamers are shown in FIG. 1A. In the preselection, 6sequences were isolated of which TA1 and TA2 (in bold) differed by asingle nucleotide. Based on the primary sequence of several of thesethioaptamers and observed variations, a pseudo-random,one-bead-one-sequence bead library of aptamers was synthesized using anautomated four-column, split-pool synthesizer as described in EnglhardtU.S. Pat. No. 7,576,037. FIG. 1B shows the sequences programmed intoeach of Columns 1-4 (CL1-4) of the synthesizer. Of these, CL1 wasprogrammed to have the addition sequence of TA1, without addition of anyX-modifications. Each * in FIG. 1B represents a pool-split step. Thefirst split occurs when beads are initially loaded into four columns,and the tenth pool occurs at completion of synthesis. Thus only nine ofthe ten total pool/split steps are indicated by an * in FIG. 1B.

The pseudo-random, one-bead-one-sequence library included ˜10⁶ (4¹⁰)one-bead one-ODN each consisting of a 30 nucleotide combinatorialsequence (9 split/pool steps) flanked by two defined primer regions atthe 5′ and 3′ ends. By “pseudo-random” it is meant that the sequencesprogrammed on the columns are derived in part from pre-selected aptamersequences. The central region contained ten parts, each of which couldhave 1 of 4 possible sequences, determined by the path each bead takesduring the split and pool method. The 3′ ends of the sequences werecovalently linked by a non-cleavable hexaethyleneglycol linker to a65-μm polystyrene bead (ChemGenes).

The unique sequence on a given bead may contain zero to 12 X-positions,but three or four Xs is most likely. In order to select highlynuclease-resistant X-aptamers, the library was prepared with a fullymonothiophosphate (permonothioated) backbone. To our knowledge, this isthe first example of the selection of an X-aptamer with completemonothioate backbone. It is noted that complete monothioate backboneaptamers are unable to be developed by the SELEX process, which requiresiterative selection to identify a target binding aptamer. Because nomore than three different alpha-S-dNTPs can be used in the enzymaticamplification steps using Taq polymerization, the required amplificationiterations of SELEX cannot select a completely monothioated aptamer.

This original pseudo-random, one-bead-one-sequence bead library servedas the base library from which a variety of additional X-aptamerlibraries were derived by conjugation with NHS-ester forms of drug-likemolecules. To select small molecule ligands as binding affinityenhancers that could be attached to the X-aptamer base library, insilico screening was carried out using AMBER (Case et al. AMBER9 (2006)UCSF) and DOCK6.4 (Lang et al. RNA 15 (2009) 1219-1230). Specifics ofthe materials and synthesis are as follows:

Materials. The dA, dG, dC and dT cyanoethylphosphoramidites, and theBeaucage reagent (3H-1,2-benzodithiol-3-one 1,1-dioxide) were purchasedfrom Glen Research. The Taq polymerase kits were from AppliedBiosystems. The TOPO TA Cloning kit was obtained from Invitrogen.Polystyrene beads (60-70 μm) with non-cleavable hexaethyleneglycollinkers with a loading of 36 μmol/g were purchased from ChemGenes Corp.(Ashland, Mass.). The oligodeoxynucleotides (ODNs) and monothioatedS-ODNs used in the study were synthesized on a 1 μmol scale in anExpedite 8909 System (Applied Biosystems) DNA synthesizer.NHS-PEG₁₂-biotin was purchased from Pierce.N-acetyl-2,3-dehydro-2-deoxyneuraminic acid (ADDA) was purchased fromSigma-Aldrich.

Synthesis of S-ODN library. Standard phosphoramidite chemistry was usedfor the synthesis of the S-ODN library. The library was prepared usingan automated four-column, split-pool synthesizer on a 1 μmol/columnscale on polystyrene beads. After first synthesizing the 3′-primers, thepseudo-random sequences were programmed, one section at a time, on thefour columns of the synthesizer to create the combinatorial S-ODNlibrary as shown in FIG. 1B. In the FIG. 1B,X=5-(aminoethyl-3-acrylimido)-deoxyuridine created by incorporation of“Amino Modifier C2 dT” (Glen Research), asterisks represent theoccurring of a split and pool synthesis step, and the primer region isshown in bold.

Subsequent addition of the 5′-primer completed the 73mer ODNs. A ‘splitand pool’ occurred at each position indicated by an asterisk in order tosynthesize the combinatorial region for the S-ODN. The example sequence(EX) in FIG. 1B is one example of an ODN that would result from a beadthat followed the column path (from 3′ to 5′) 3-4-2-4-1-3-2-4-2-1,depicted as underlined parts, during the split-pool method. Thestep-wise coupling yields were approximately 99% as determined by thedimethoxytrityl cation assay. Sulfurization chemistry utilized theBeaucage reagent. The S-ODN combinatorial libraries on non-cleavablelinker beads were deprotected with concentrated ammonium hydroxide at37° C. for 21 h. and then washed with doubly distilled water.

Expression and Purification of CD44-HABD. DNA encoding CD44-HABD (aminoacid residues 20-178) was synthesized and cloned into expression vectorpET19b between the NdeI and BamHI sites. Protein expression, refoldingand purification followed published procedures (Banerji, S.; et al.Protein Expression and Purification 14 (1998) 371-381). The purity ofCD44-HABD was analyzed by gel electrophoresis (SDS-PAGE and nativePAGE). The concentration of CD44-HABD was determined by UV-VisSpectrophotometry (276 nm, ε=12.95 mM⁻¹ cm⁻¹) and BCA assay (Pierce).

Labeling CD44-HABD protein with biotin. To 130 μL of CD44-HABD (40.5 μM)in PBS was added 1.2 μL of NHS-PEG₁₂-biotin (125 mM in DMF). Thereaction was incubated on ice for two hours. Biotin labeled protein waspurified from non-reacted biotin reagent using Zeba desalt spin columns(Thermo Scientific). The labeled protein was stored at 4° C.

Conjugation of ADDA to XA bead library. To 1 μL of a 1 molar ADDAsolution in DMSO, 5 μL of a freshly prepared ethyldimethylaminopropylcarbodiimide (EDAC) solution (1 molar in H₂O) and 5 μL of anN-hydroxysuccinimide (NHS) solution (1 molar in DMSO) were added. Thereaction was kept at room temperature for one hour. To couple ADDA tothe original X-aptamer bead library, 10% of the original XA library wasadded, and the resulting mixture was gently shaken overnight. Beads werewashed to remove unused reagents, and the conjugation was confirmed by aninhydrin test.

Selection of the ADDA-modified X-aptamer library against CD44-HABD. Thescreen began with the ADDA X-aptamer library incubated in PBS (pH 7.4)containing Tween 20 (0.1%, v/v) and bovine serum albumin (BSA, 0.1%) forone hour, with shaking, to block non-specific protein binding. Thelibrary was then washed with PBS (pH 7.4). A negative selection wascarried out first by incubating the washed X-aptamer beads withstreptavidin-coated Dyna beads (Invitrogen) for two hours at roomtemperature. The slurry of the beads mixture was allowed to slowly passin solution near a magnet to remove the Dyna beads and any X-aptamerbeads which have the Dyna beads bound nonspecifically on their surface.The rest of the X-aptamer beads were then suspended in a dilute solutionof biotinylated CD44-HABD (0.01 nM in PBS, pH 7.4) at room temperatureovernight. After washing with PBS containing 0.1% Tween-20 and BSA, PBScontaining 0.1% Tween-20, and PBS, the library was incubated withstreptavidin-coated Dyna beads for two hours at room temperature. Thebeads were washed thoroughly with PBS containing 0.1% Tween-20 and BSA,PBS containing 0.1% Tween-20, and PBS. The positive X-aptamer beads,which had CD44-HABD bound tightly, were isolated from the incubation bypassing the beads in solution near a magnet. The positive beads weretransferred onto a glass microscope slide and selected manually bypipette. To remove bound proteins, each positive bead was incubated in 8molar guanidine hydrochloride for one hour then rinsed ten times withwater.

One-bead one-PCR amplification and sequencing of PCR products. Thewashed beads were directly used for the ‘one-bead one-PCR’ amplificationprocess using the 5′ end and the 3′ end primers shown below:

5′ PRIMER: 5′-GAGATTCATCACGCGCATAGTC-3′ SEQ. ID. 25 3′ PRIMER:5′-CGACTATGCGATGATGTCTTC-3′ SEQ. ID. 26

A selected single bead was mixed with the following PCR components: 6 μLof 25 mM MgCl₂, 0.5 μL of Taq polymerase (5 U/μL), 1 μL of 8 mM dNTP, 10μL of PCR buffer, and 1 μL of 40 mM primers. The PCR was run on aGeneAmp PCR system 2400 (Perkin Elmer). The PCR mixtures were thermalcycled using the following scheme for amplification: 94° C. for 5 min (1cycle); 94° C. for 1 min, 40° C. for 2 min and 72° C. for 1 min 10 sec(35 cycles); 72° C. for 10 min (1 cycle). The PCR product was insertedinto pCR2.1 TOPO TA vector (Invitrogen) and sequenced. PCR amplificationconverts the original X modifications to Ts. X positions were determinedby reference to the original library design (FIG. 1B), using theadjacent bases as “barcoding”. FIG. 1C lists the XA sequences obtained(XA1-13, SEQ ID NOs 12-24). Typically the derived XA will beresynthesized off the non-cleavable linker beads for verification andthen conjugation with desired ligands.

Conjugation of ADDA to XAs in solution. To 1 μL of a 1 molar ADDAsolution in DMSO, 5 μL of a freshly prepared EDAC solution (1 molar inH₂O) and 5 μL of an N-hydroxysuccinimide (NHS) solution (1 molar inDMSO) were added. The reaction was kept at room temperature for onehour. To couple ADDA to XAs in solution (not on beads), XAs (0.05 μmol)were dissolved in 100 μL of sodium bicarbonate buffer (0.1 molar, pH8.5). ADDA-NHS ester solution was added to the solution of XAs. Themixture was first vortexed and then shaken at room temperature for fourhours. The ADDA-modified XAs were purified by reverse phase HPLC on aHamilton PRP-1 column. Buffer A: 100 mM triethylammonium acetate in H₂O(pH 7.5); buffer B: acetonitrile. Gradient: 0-40% B, 0-60 min; 40-100%B, 60-70 min; 100-0% B, 70-75 min. Flow rate was 2 mL/min. The fractionsof ADDA-modified XAs were combined, lyophilized and analyzed by 15%PAGE.

¹H,¹⁵N-HSQC NMR Data for CD44-HABD. ¹H,¹⁵N-HSQC data were collected onan 800 MHz Varian UnityPlus spectrometer (Rice University) equipped witha cryogenic probe using 32 transients per fid, sweep widths of 3.2 kHzand 11.2 kHz for f1 (¹⁵N) and f2 (¹H), respectively, and 128 and 1912complex data points for f1 and f2, respectively. Data were processedusing VNMRJ (Varian, Inc.) or Felix (Felix, Inc.) software. The protondimension was referenced to TSP and nitrogen shifts were referencedindirectly. CD44-HABD was prepared in the NMR buffer (20 mM Tris, 50 mMNaCl, 10% (v/v) D₂O, pH 6.7). ADDA was dissolved in DMSO (1.0 M) asstock solution. CD44-HABD (70 μM) with ADDA (7 mM) was prepared in thesame buffer (20 mM Tris, 50 mM NaCl, 1.25 mM ADDA, 0.7% (v/v) DMSO, 5%(v/v) D₂O, pH 6.7).

ADDA binding to CD44-HABD. CD44-HABD was prepared in the NMR buffer (20mM Tris, 50 mM NaCl, 5% (v/v) D₂O, pH 6.7). ADDA was dissolved in DMSO(1.4 M) as stock solution. The ADDA sample for NMR was prepared in thesame buffer (20 mM Tris, 50 mM NaCl, 1.25 mM ADDA, 0.1% (v/v) DMSO, 5%(v/v) D₂O, pH 6.7). CD44-HABD (27 μM) with various concentration of ADDA(14 μM, 25 μM, 50 μM, 250 μM and 1250 μM) was also prepared in the samebuffer. The ¹H-NMR spectra of CD44-HABD only, ADDA only and CD44-HABD inpresence of ADDA were obtained.

The equilibrium dissociation constant of ADDA to CD44-HABD wasdetermined by ¹H-NMR. FIG. 2A shows a set of spectra measured onchanging the concentration of the ADDA ligand, whereI_(lig)/I_(lig-free) represent the ratio between the intensity of thesignal of the protein CD44-HABD with and without the ligand. The ¹H-NMRspectrum of the ligand alone showed no signal in the region of interestthat could have interfered with the signal intensity ratios. Severalpeaks were selected (7.7925, 7.9025, 7.9225, 7.9375, 7.9575 and 7.9925ppm) in order to obtain a statistical value of the dissociation constantof the ligand ADDA, K_(D)=2.22±0.82 mM, as shown in FIG. 2B andsummarized in the following Table 1:

TABLE 1 DISSOCIATION CONSTANT OF THE LIGAND ADDA peak (ppm) 7.79257.9025 7.9225 7.9375 7.9575 7.9925 Average K_(D) K_(D) [mM] 3.08231.4438 1.7213 1.9167 1.7083 3.4259 2.22 ± 0.82

Filter binding assay. The equilibrium binding constants of selected XAsfor CD44-HABD were determined by a filter binding assay. Thebiotinylated XAs (1 nM) were incubated with varying concentrations ofCD44-HABD in 50 μL of 20 mM Tris, pH 8.0, 150 mM NaCl for 40 min at roomtemperature and then transferred to a 96-well dot-blot apparatus andfiltered under vacuum onto nitrocellulose membranes, which retain theCD44-HABD along with any bound XAs. The amount of biotinylated XAretained at each spot was determined by chemiluminescent detection usingthe Chemiluminescent Nucleic Acid Detection Module (Thermo Scientific)following the manufacturer's instructions. The chemiluminescent signalswere collected on a Chemimager (Alpha Innotech). Image analysis andquantification of spot intensities were performed using ImageJ (version1.42q). Binding analysis was based on the spot intensities on thenitrocellulose membranes with subtraction of background spot intensitydue to the buffer effect from all the data points. Saturation bindingcurves were generated by using GraphPad Prism with curve fits assuming asingle binding site. The equilibrium dissociation constants, K_(D), werederived from these curves and are listed in Table 2. The NH₂ XA columnlists K_(D)s for XAs where X=amino-dU. The ADDA XA column lists K_(D)sfor XAs where X=ADDA-dU.

TABLE 2 EQUILIBRIUM DISSOCIATION CONSTANTS OF SELECTED PERMONOTHIOATEDXAS TOWARD CD44-HABD K_(D) (nM) Name NH₂ XA ADDA XA XA1  62.9 ± 10.3108.7 ± 15.4 XA2  55.5 ± 13.4  78.3 ± 18.1 XA3 110.9 ± 18.5 139.6 ± 16.8XA4 137.4 ± 37.4 102.7 ± 16.8 XA5 137.4 ± 37.4 124.8 ± 38.4 XA6 213.9 ±31.6 150.0 ± 25.7 XA7 305.8 ± 69.3 116.3 ± 20.7 XA8 463.2 ± 82.4 274.0 ±54.3 XA9 476.9 ± 89.5  555.5 ± 108.1 XA10  449.9 ± 180.3 N.A. XA11 845.6 ± 151.4 1086.0 ± 390.5 XA12  962.6 ± 182.7  878.7 ± 134.9 XA131377.0 ± 237.0  312.5 ± 68.53

The binding curves of ADDA XA3 and ADDA XA7 are shown in FIG. 3A andFIG. 3B, respectively. MFold-predicted secondary structures indicatedthat all selected XA sequences can form hairpin loop structures with therandom region forming the loop and the primer regions making up the stemregions (FIGS. 4A and B). Primer regions are in black while randomregions are circled. The sequence of the 5′ primer is SEQ. ID. 25, whilethe sequence of the 3′ primer is SEQ. ID. 26. Proposed binding motifsare encircled with a solid line. The positions where ADDA was coupledare also shown. From these binding motifs based on the predictedsecondary structures, several smaller constructs were made of variousstem-loop regions of these selected X-aptamers (EXAMPLE 2).

Molecular Dynamics Simulation protocol. The initial structure (humanCD44-HABD, 1POZ.pdb) was neutralized by adding Na⁺ counter ions using analgorithm of xLeap (AMBER9 suite of programs). The latter structure wasthen surrounded by a 10 Å layer of TIP3P model water molecules in anorthorhombic box of approximately 60×72×90 Å containing a total of 28386atoms. The system was minimized by 1000 steps steepest descent methodthen 4000 steps of conjugate gradient. The solvated system was thenequilibrated as follows i) 40 ps MD simulation to gradually heat thesystem from 0 to 300K keeping all molecules restrained by 25 kcal mol⁻¹Å⁻² under NVT conditions, ii) 20 ps MD simulation at 300K with allmolecules restrained by 25 kcal mol⁻¹ Å⁻² under NVT conditions, iii) thesystem was subsequently equilibrated in 7 MD simulation rounds over 1180ps where the positional restraints were gradually relaxed under NPTconditions at 300K and finally a structure production run of 10 ns MDsimulation at 300K under NPT conditions was performed. The long-rangeelectrostatics were accounted for using the particle-mesh Ewaldsummation method, as implemented in the PMEMD module of AMBER9, and theforce field ff99SB was applied. The SHAKE algorithm was used toconstrain covalent bonds to hydrogen atoms allowing a time step of 2 fs.A cutoff of 9 Å was chosen for the non-bonded van der Waalsinteractions. During the heating protocol at NVT conditions, theBerendsen temperature coupling algorithm was used with a couplingconstant of 2.0 ps. During the equilibration and production of thesimulation, the Langevin dynamics were used with a collision frequencyof 1.0 ps⁻¹.

This simulation allowed the identification of 4 representativestructures using the cluster analysis algorithm on ptraj software(AMBER9 suite of programs). A second 10 ns MD simulation was performedincluding the hyaluronic acid ligand bound into the HABD this time. Thesame MD protocol was used as described before but applying theff99SB+GLYCAM06 force field because of the nature of the hyaluronic acidligand. Therefore, a second set of 4 representative structures werefound using the cluster analysis algorithm on ptraj software (AMBER9suite of programs). These 8 representative structures plus the original1POZ.pdb structure were used as possible binding pockets for a virtualscreening of small molecules.

Virtual Screening protocol. A group of 2553 possible ligands withmodifiable groups allowing attachment of the ligand to the X-aptamer ODNscaffold were selected from ChemBridge database and 245 ligands fromSigma-Aldrich. Each ligand followed a 0.5 ns MD simulation on implicitwater using the Generalized Born method at 300K in order to find andselect an ensemble of possible rotamers. The rotamers were chosen usingthe following protocol: i) 20 ps to gradually heat the system from 0 to300K, ii) 460 ps of production run at 300K and iii) 20 ps to graduallycool down the system from 300 to 0K. A total of seven rotamers perligand were initially selected: the first rotamer was the initialstructure of stage i) which is the original structure downloaded fromthe database, five more rotamers were found using the cluster analysisalgorithm on ptraj software (AMBER9 suite of programs) from stage ii)and the final rotamer was selected at the end of stage iii). Allrotamers were minimized with the Generalized Born method. Onlyrotamers >2.0 rmsd from the original structure (first rotamer) wereconsidered.

After rotamer selection a semi-empirical quantum calculation (AM1-BCCmethod, AMBER9 suite of programs) was performed for each rotamer, and anaverage charge was calculated and used for all rotamers of the sameligand. The initial rotamer structures and their charges are criticalcomponents during the virtual screening process in order to get goodlead compounds. Using DOCK6.4 software on the 9 representative proteinstructures with the 2798 small molecules, the ADDA ligand(N-acetyl-2,3-dehydro-2-deoxyneuraminic acid, PubChem: CID 65309) wasselected as lead compound. In one example of the results of a similarprocess for ligand selection, FIG. 7A depicts the structure of a 7-merhyaluronic acid (HA) built by xLeap (AMBER9 suite of programs), FIG. 7Bdepicts the structure of the ADDA ligand selected by the virtualscreening protocol and FIG. 7C shows a docking overlay of HA (bondrepresentation) with ADDA ligand (circled) on the CD44-HABD.

EXAMPLE 2 Second Generation X-Aptamers

Secondary structure predictions performed using MFold (Zuker, M. NucleicAcids Research 31 (2003) 3406-3415) suggested that all selected XAsequences can form hairpin loop structures in which the random regionsform loops and the primers form stem regions as shown in FIGS. 4A-C.Based on these predicted structures, several binding motifs and smallerconstructs of various stem-loop regions were identified. The sequencesof these truncated XA-sequences are shown below in Table 3. Suchtruncation allows elimination of excessive X moieties if desired whilekeeping the ligand binding domain.

TABLE 3 SEQUENCES OF TRUNCATED X-APTAMERS Motif Motif Sequence SEQ IDCL.1 5′PRIMERCCAA GGCC TGC AAG GGA ACC AAG 7 GAC AC AG 3′PRIMER 1 (XA2)5′- AAG GGA ACC AAG GAC AC TA C-3′ 27 2 (XA1) 5′- C XGX TAG GGA ACC AAGACG A-3′ 28 3 (XA4) 5′- GCC TGC AAG ACG XCC ATA GAC AC-3′ 29 4 (XA9) 5′-GATC TGC AAX GTA ACC ATA GAC A-3′ 30 5 (XA5) 5′-A GAXA CAG TAA ACG XCCATA GAC AC-3′ 31 Alternating underlined bold letters correspond to thesplit-pool sequence sections in FIG. 1B for these aligned sequences.Following each motif number in column 1 above is the name of the parentaptamer of FIG. 1C from which the truncation was derived

The equilibrium binding constants of the small constructs weredetermined by the filter binding assay. The binding curves of exemplarymotif 3, motif 4 and motif 5 are shown in FIGS. 5A-C respectively. Thederived equilibrium binding constants of the above small XA constructsare shown below in Table 4.

TABLE 4 EQUILIBRIUM BINDING CONSTANTS OF THE SMALL XA CONSTRUCTS OFTABLE 3 Dissociation constant (nM) Full-length parent sequence Cl.1, 191± 25 partially monothioated Full-length parent sequence CL.1 230 ± 47permonothioated Phospho X-aptamers Thiophospho X-aptamers Motif X =NH₂-dU X = ADDA-dU X = NH₂-dU X = ADDA-dU 1 10.3 ± 1.3 15.0 ± 2.0 2 43.1± 9.5 2.0 ± 0.6 48.0 ± 18.0 15.5 ± 3.2 3 27.6 ± 3.5 19.5 ± 3.2  81.2 ±30.9  64.8 ± 13.7 4  6.8 ± 1.8 2.1 ± 0.2 35.4 ± 7.4  13.6 ± 3.0 5 13.8 ±4.1 3.9 ± 1.0 18.0 ± 3.7  10.1 ± 2.6 ADDA-dU is the ADDA adduct with5-(aminoethyl-3-acrylimido)-deoxyuridine. Motif 1, which contains no X,was included to compare the phosphoaptamer and the thioaptamer forms.

Remarkably, coupling ADDA with smaller stem-loop constructs from thebest X-aptamer sequences, motifs 2 and 4 (ADDA adduct) have ˜2 nMaffinity to CD44-HABD, which is an increase in binding affinity of˜115-fold between the full-length permonothioated parent sequence andthe final ADDA-conjugated XA (phosphoform). Moreover, theADDA-conjugated permonothioated motif 5 has ˜10 nM affinity, which is anincrease of ˜23-fold compared to the permonothioated parent sequence. Inevery case in Table 4, the ADDA-conjugated XA showed increased affinitycompared to the unconjugated XA. In the best case, ADDA conjugationincreased affinity ˜22-fold. ADDA is a weaker binder (˜2 mM) toCD44-HABD. By conjugation to an aptamer, the binding affinity of ADDAmodified XAs has improved the affinity 1 million fold (from 2 mM to 2nM).

By introducing a protein binding small drug molecule, ADDA, into the5-position of dU residues at random positions of the aptamers and/orreplacing one of the non-bridging phosphate oxygen atoms with sulfuratoms, we are able to select an X-aptamer with <10 nM affinity toCD44-HABD through a non-iterative bead-based selection from largecombinatorial libraries of X-aptamers. The present bead-based method iscompatible with both monothiophosphate- and dithiophosphate-modifiedthioaptamers, and even complete monothiophosphate modification of thebackbone, as reported here, which is not possible with traditional SELEXmethods. In addition, the present method allows a given base, such asdT, to be replaced with a modified version of that base, such asamino-dU or ADDA-dU, in only a subset of positions, while the remainingdT positions remain unmodified. This is not possible with SELEX, whichcan only incorporate certain modified bases, and only by totalreplacement of the natural base with the modified version. Furthermore,the present method requires only one or two rounds of aptamer selection,in contrast to the 10-15 rounds typical in traditional SELEX.

As expected, the effect of ADDA as a binding affinity enhancer islocation-dependent within the aptamer. The process of finding theoptimal position of the ligand was part of the X-aptamer selection sinceADDA was attached to the aptamers at various positions. Bysimultaneously selecting the optimal sequence of the aptamer scaffoldand the orientation and position of the small drug presented by theoptimal scaffold, enhanced affinities were achieved. The incorporationof ADDA not only expands the XA's chemical diversity but also thesurface area of binding, thus the XA can also offer enhancedspecificity.

While the present example describes conjugation with one specific drugat a time, multiple drug hits can be randomly attached as well, toprovide enhanced combinations of binding moieties. More than one ligandcan be attached by pausing the DNA synthesis for the addition of aligand and then continuing the DNA synthesis and coupling reactions. Byusing two or more chemical linkers in one root library, multiple drugscan be selectively incorporated. The present methodology can be appliedto most target proteins with a variety of small molecules to createhighly chemically modified X-aptamers that have the combinedcharacteristics of drug molecules, proteins and nucleic acids.

Cell Binding Assay. Ovarian cancer cell line IGROV (CD44+) was incubatedwith fluorescently labeled motif 3 (amino form and ADDA adduct) at 37°C. for 2 hours. After being washed 2 times, the cells were subjected toflow cytometry analysis. The flow cytometry histogram of FIG. 6 showsthat the binding of motif 3 to IGROV cells is improved by theconjugation of ADDA ligand. The result clearly shows that theconjugation of ADDA to motif 3 improves its binding to IGROV cells.

Use of Thioaptamers in vivo. As just one example, FIG. 16 shows a nearinfrared image of a mouse with a human pancreatic cancer xenograph(circled) in which an IR800dye-labeled thioaptamer accumulates in thetumor. FIG. 17 shows the predicted two dimensional structure of theESTA-1 thioaptamer sequence (SEQ. ID. 32). This in vivo test demonstratethat thioaptamers are sufficiently stable in vivo to survive thenumerous circulatory cycles required for accumulation in a tumor andprovides substantiation for their applicability to diagnosis andtreatment of disease including cancer.

EXAMPLE 3 Click Chemistry

In another embodiment, libraries are constructed using click chemistryas an alternative to the amide coupling chemistry to introduce multiple,in silico selected, drug-like hits into the X-aptamers. Click chemistryis used to describe two step chemical reactions that involvecopper-catalyzed triazole formation from an azide and an alkyne as shownin FIG. 8. As shown, the azides (R-N3) react with alkynes under coppercatalysis to form triazoles by cycloaddition. The azide and alkynemoieties can be used interchangeably and either one can be used to tagthe molecule of interest, with the other used for subsequent detection.The azides and alkynes are biologically unique, inert, stable, andextremely small and can be used to tag nucleotides, which remainacceptable substrates for the enzymes. X-aptamers have also beenchemically synthesized to contain 5-ethynyl-dU, which, when reacted withX-azides, forms a triazole appended to the X-aptamer at the 5-X-dUresidue. If only one type of X-azide is to be attached to a randomaptamer, a library is created in which each position is a standard ODNbase or a 5-alkynyl-dU. If the alkyne is not protected, the azides canbe added before or after deprotection of the DNA. Addition beforedeprotection of DNA allows for directed attachment of several, possiblydifferent, azide molecules to the ODN library at any position. Tointroduce the alkyne label, either thymidine-like alkynylphosphoramidite building block 1(5′-Dimethoxytrityl-5-ethynyl-2′-deoxyuridine,3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite)or 2(5′-Dimethoxytrityl-5-(octa-1,7-diynyl)-2′-deoxyuridine,3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite)in FIG. 9 was incorporated into the oligonucleotides in Table 5 belowusing standard phosphoramidite chemistry. The azides are subsequentlyattached by click chemistry as shown in FIGS. 10A and B (showing thereaction on a bead). The coupling yield was excellent.

The click reaction on solid support was performed by shaking the resin(ODN-10, Table 5) with a solution of CuBr,tris(benzyltriazolylmethyl)amine (TBTA), sodium ascorbate, and the azide3 (7-(Diethylamino)coumarin-3-carbonyl azide) (building block 3 of FIG.9) or the azide 4-fluorescent dye Alexa® Fluor 555 azide (Cat. No.A20012, available from Life Technologies) (building block 4). Afterreaction, a portion of resin was packed in a new column and synthesiscontinued to yield ODN-11. The DNA was finally cleaved from the resin,and all protecting groups were removed by exposing the resin to ammonia.

TABLE 5 ODNS 1-11 EMPLOYED IN THE “CLICK” CHEMISTRY SEQ. ODN SequenceID.  1 5′-CGGCYGTTCATTYGGC-3′ 33  2 5′-CGGCTGTTCATTYGGC-3′ 34  35′-CGGcYGTTCATTYgGC-3′ 35  4 5′-CGGCT G TTCATT Y GGC-3′ 36  5 5′-CGGCT GTTCATT Y GGC-3′ 37  6 5′-TACGXCTCGXAGTA-3′ 38  7 5′-TaCGXCTCgXAGTA-3′ 39 8 5′-GGGGCACGTTTATCCGTCCCTCCTAGTGGCGXGCCCC-3′ 40  95′-GGGGCXCGTTTATCCGTCCCTCCTAGTGGCGTGCCCC-3′ 41 10^(a) 5′-XAGTA-3′ ll^(b)5′-TGTCTTGCcTCGGTTTtCgCTGTTgTCgTCCgCtTTCG 42 TTCXAGTA-3′ X = DNAnucleotide based on phosphoramidite 1; Y = DNA nucleotide based onphosphoramidite 2; a, t, c and g indicate 5′-monothioate linkage; G andY indicate 5′-dithioate linkage. ^(a)Click reaction performed on resin.^(b)Oligonucleotide synthesis continued (adding 41 more bases) afterclick reaction performed on ODN-10.

The click reaction was performed in solution after oligonucleotidedeprotection. Treatment of the ODNs-1-9 with concentrated NH₃ cleavedthe DNA from the resin. Under these conditions the base protectinggroups were removed as well. The obtained DNA, bearing free alkynes, wassubjected to the click reaction in solution (CuBr, TBTA, the azide 3 or4), yielding the modified DNA. The modified ODNs, the products fromclick reaction, were analyzed by HPLC, mass spectrometry (MS), andPAGE-gel analysis. FIG. 11A shows HPLC traces for the crude clickreactions of ODN-2 with azide 3 and 4. The click reactions have yieldedgreater than 90% as determined by the peak area. The click reactionproducts from these two reactions have longer retention times than freeODN-2 because a large hydrophobic molecule (azide 3 or 4) is conjugated.The click reaction products were analyzed by polyacrylamide gelelectrophoresis as shown in FIG. 11C. The ODN-2 azide adducts arefluorescent without ethidium bromide staining after modification withazide 3 or 4. The identity of the click reaction products were confirmedby mass spectrometry (FIG. 11B). ODN-2 m/z: calculated 4965.3, observed4969.6; ODN-2-Azide-3 adduct m/z: calculated 5251.6 observed 5229.8;ODN-2-azide-4 adduct m/z: calculated 5711.0, observed 5710.2.

DNA synthesis can be halted for the addition of a small molecule azideto a 5-alkynyl-dU base while the DNA is still attached to a CPG bead,and synthesis can be restarted with subsequent additions later in thesequence. This method allows for the directed attachment of several,possibly different, azide molecules to the ODN library at any position.

Thus enormously more complex libraries of X-aptamers may be created inwhich every base can have an amino-acid-like side-chain or a complexdrug moiety. Rather than only use 4 bases or even 20 amino acid-likeside-chains, virtually an unlimited range of chemical functional groupscan be introduced into an X-aptamer that can fold into a unique 3Dscaffold to present to the target protein multiple drug-like groups withan enormously more complex range of substituents.

EXAMPLE 4 Selection of X-Aptamer with Two Nucleotide Ligands

Considering the results of the equilibrium binding constants determinedby the filter binding assay (Table 4), Motif 2 was selected in order tofollow several molecular dynamics simulations of possible bindingconformations interacting with CD44-HABD. The software AMBER12 (Case etal. AMBER12 (2012) UCSF) was used with the Molecular Dynamics Simulationprotocol previously described herein.

Once the best binding model was obtained after the simulations, it wasdetermined that a new pocket was accessible close to the cytosine 13(SEQ ID 28, Table 3) and facing the major groove side. Considering thatthis cytosine is part of the G-C base-pair stabilizing the hairpin ofMotif 2 (FIG. 4A, XA1), which plays an important role for the stabilityof the binding interaction, it was decided select a cytosine analogue asa new ligand in order to keep the G-C base-pair intact. The followingthree possible deoxycytidine analogs were selected based on the abilityto base pair faithfully with dG with virtually no disruption of thenormal duplex structure.

The deoxycytidine analogs shown below were obtained from Glen Research:

Acronym Structure Chemical Name tC

tC-CE Phosphoramidite: a.k.a. 5′-O-(4,4′-Dimethoxytrityl)-1′-(1,3-diaza-2-oxophenothiazin-1-yl)-2′-deoxy-B-D- ribofuranosyl-3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite tCo

tC °-CE Phosphoramidite: a.k.a. 5′-O-(4,4′-Dimethoxytrityl)-1′-(1,3-diaza-2-oxophenoxazin-1-yl)-2′-deoxy-B-D- ribofuranosyl-3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite tCnitro

tCnitro-CE Phosphoramidite: a.k.a. 5′-O-(4,4′-Dimethoxytrityl)-1′-(7-nitro-1,3-diaza-2-oxophenothiazin-1-yl)-2′- deoxy-B-D-ribofuranosyl-3′-[(2-cyanoethyl)-(N,N-diisopropyl)]- phosphoramidite

The geometry of the three ligands were initially optimized withGaussian03 (Frisch et al. 2004) using HF/6-31G* level of theory and thena single-point calculation (with the same level of theory) was performedto obtain the electrostatic potential. Fitting charges to theelectrostatic potential were then performed with RESP (AMBER12 suite ofprograms). These new ligands were then incorporated into the model(CD44-HABD+motif2-ADDA-cytosine_analogue) and each model was followed bya molecular dynamics simulation of 50 ns. After these calculations, anX-aptamer model having ADDA and tCnitro ligands was determined to bemost stable by molecular modeling.

EXAMPLE 5 Selection of X-Aptamer with Multiple Binding Moieties

The previous examples provide herein have shown that a small moleculedrug (ADDA) can be randomly incorporated along with modified nucleotidesinto an X-aptamer. While the prior examples describe conjugation withone specific drug at a time, multiple drug hits can be randomly attachedas well, to provide enhanced combinations of binding moieties. Forexample, more than one ligand can be attached by pausing the DNAsynthesis for the addition of a ligand and subsequent DNA synthesis andcoupling reactions. By using two or more chemical linkers in one rootlibrary, multiple drugs may be selectively incorporated. Thismethodology can be applied to most targets with a variety of smallmolecules to create highly chemically modified X-aptamers that have thecombined characteristics of drug molecules, proteins and nucleic acids.

For example, both amidites with 5-dU ethynyl groups may be used tocouple to small molecule azide leads with click chemistry as well as5-amino dU phosphoramidites, which can be coupled with small moleculecarboxylate leads with amide coupling reagents. The 5-dU ethynyl groupsand 5-amino dU groups may be randomly placed into the X-aptamer backboneand stepwise conjugating both the azide leads and carboxylate leads tothe bead library will produce an XA library with both leads (1-6 azides,1-6 or more carboxylates on each oligonucleotide strand). Alternativelythe DNA synthesizer can be stopped during a pooling step and clickchemistry performed with different azide drug leads and resplitting thebeads onto the synthesizer for further elongation and placing >2different drugs on each XA. The goal of linking two or more drugfragments has been elusive because it is not possible to structuralidentify the best linker to attach the two drug fragments so that properspacing and orientation is achieved. The XA method described hereinavoids that problem since combinatorial chemistry can be used and theoligonucleotide scaffold allowed to form secondary and tertiarystructural motifs that allow the most favorable separation andorientation of the multiple drug fragments.

EXAMPLE 6 Generation of Small Molecule-Aptamer “Two-Hit” Conjugates

As shown in FIG. 18, in one embodiment a small molecule library isinterrogated in silico by screening with computer docking of each memberof the library (potentially millions) onto various binding sites of theprotein. The tightest binding theoretical hits are selected and bindingto the target protein (i.e. HABD CD44) is confirmed, such as by NMR. Forexample, ¹⁵N labeled protein can be used so that a 2D HSQC ¹⁵N-¹H NMRspectrum can be used to identify the individual amides signals that areperturbed in the presence of the drug. In the 3D backbone model inset toFIG. 18 those residues are highlighted that show the largest chemicalshift perturbations upon drug binding—signifying the drug binding site(ADDA drug is circled). This validates the original model where the drug(circled) is shown in the space filling model.

As described previously herein, an XA library is then constructed withapproximately 1,000,000 different sequences and different locations andnumbers of drugs (i.e. ADDA molecules) attached to the beads. The beadlibrary is selected and individual beads sequenced and K_(D) measured.Optimization of the selected sequences is performed by modeling of thetwo and three dimensional structures of the binding domains andshortened motif sequences are prepared. In the example provided hereinfor CD44 binding, the resulting drug modified shortened aptamers showedmuch tighter binding to the protein, enhancing drug binding by 1,000,000fold and aptamer alone binding by 100-fold. Where there is no entropicloss in orienting the conjugated drug and the linked oligonucleotide tothe protein, it is predicted that the conjugate will attain2×10⁻³×200×10⁻⁹ or 0.4 nM binding affinity for the conjugate. Remarkablythe actual binding affinity attained was 2 nM vs. the theoretically bestaffinity of 0.4 nM.

EXAMPLE 7 Dual-ligand CD44 X-aptamer Motifs

In order to enhance the affinity and selectivity of our next generationX-aptamers, by adding drug-like ligands or amino-acid like side chainsto the bases, we devised a bead-based combinatorial library selectionmethod in which one or more X-ligands can be linked to aptamers in arandom fashion. By combining the one-bead, one-sequence thioaptamerselection method with the incorporation of pseudo-randomly placed basesthat contain chemical linkers, additional binding X-ligands can beappended onto aptamers or thioaptamers to create a next-generation,X-aptamer library, and the best binding X-aptamers can be selected fromthis large pool of sequences. In one example, ligands were attached withamine-N-hydroxysuccinimide (NHS)-ester coupling to alter the bindingproperties of a thioaptamer specific to the CD44 hyaluronic acid bindingdomain (CD44-HABD). We have shown that such X-aptamers had strongbinding to human cells expressing CD44 (found on stem cells and cancercells) and showed the same pattern as that of anti-human CD44 antibody.To this end selected CD44 X-aptamers (motif 3 and motif 5 of Table 6)were shown to bind to human ovarian cancer IGROV cells overexpressingCD44. Cultured IGROV cells were incubated with Cy3-X-aptamers or FITCconjugated anti-human CD44 antibody. A blue nuclear counterstain wasdone by Hoechst 33342 and both motifs bound to membrane of IGROV cellswith the same pattern as that of CD44 antibody (data not shown). TheseX-aptamers were selected with a small drug-like molecule attachedrandomly to the bead-based library. The drug ADDA(N-acetyl-2,3-dehydro-2-deoxyneuraminic acid) was selected as the leadcompound because it bound in the hyaluronic acid (HA) binding pocketbelieved also to be the binding site of the originally selectedthioaptamers.

Based on molecular dynamics calculations, tCnitro-CE Phosphoramidite wasincorporated into the motif 2, motif 3 and motif 5 X-aptamers as anotherligand for CD44 HABD (Table 6). We made 11 constructs with a C9 spacerand Cy3 at the 5′ end. Each of them contains two different ligands,amino C2 dT (to which the ADDA drug is attached) and dCnitro.

TABLE 6 SEQUENCES OF X-APTAMER MOTIFS TESTED BY CELL BINDING ASSAY SEQ.ID. Sequence for dual-ligand X-aptamer motifconstructs based upon motifs 2, 3 and 5 VI59A 5′-CTGXTAGGGAACYAAGACGA-3′43 VI59B 5′-CTGTXAGGGAACYAAGACGA-3′ 44 VI59C 5′-CTGXTAGGGAACYAA-3′ 45VI59D 5′-CTGTXAGGGAACYAA-3′ 46 VI59E 5′-CTGTXAGGGAAYCAAGACGA-3′ 47 VI59F5′-CTGTXAGGGAAYCAA-3′ 48 VI59G 5′-GCGTGCAAYACCXCCATAGAGAC-3′ 49 VI59H5′-GCCTGCAAGYAGXCCATAGACAC-3′ 50 VI59I 5′-GCCTGCAAGAYGXCCATAGACAC-3′ 51VI59J 5′-GCCTGCAAGACYXCCATAGAGAC-3′ 52 VI59K5′-AGATACAGTAAAYGXCCATAGACAC-3′ 53 Sequences of the original motifs 2-5Motif 2 5′-CXGXTAGGGAACCAAGACGA-3′ 54 Motif 35′-GCCTGCAAGACGXCCATAGACAC-3′ 55 Motif 4 5′-GATCTGCAAXGTAACCATAGACA-3′56 Motif 5 5′-AGAXACAGTAAACGXCCATAGACAC-3′ 57 Note: X = amino modifierC2 dT; Y = tCnitro-CE phosphoramidite

The X-aptamer motifs of Table 6 were incubated with CD44 positiveovarian cancer cell line IGROV at 37° C. for 2 hours with the resultshown in FIG. 19. Mean fluorescent intensity and % Gated cells of 10,000cells were used to compare the relative binding affinity towards the newXA motifs: AMS: all monothioate backbone substitution; ADDA:N-Acetyl-2,3-dehydro-2-deoxyneuraminic acid (coupled to the motifs viaamino modifier C2 dT).

Dithioate CD44 X-aptamer Our previous study indicated that X-aptamermotif 3 and motif 5 had strong binding to human ovarian cancer IGROVcells and shown the same pattern as that of anti-human CD44 antibody. Weinvestigated the effect of a dithioate substitution replacing bothnonlinking phosphoryl oxygens with sulfurs. Such modification ofoligonucleotides can confer nuclease resistance as well as lead to theenhanced binding affinity. We made 14 dithioate X-aptamers (2 to 5dithioates incorporated at various positions) based on the sequences ofmotif 3 and motif 5 (Table 6). The X-aptamer motifs (Table 7) wereincubated with CD44 positive ovarian cancer cell line IGROV at 37° C.for 2 hours. Mean fluorescent intensity and % of positive cells of total10,000 cells were used to compare the relative binding affinity amongthe XA motifs. Incubation with Amino-dT XAs are shown in the first barof each set. Incubation with ADDA modified dithioate XAs are shown inthe second bar of each set. In FIG. 20, the new sequences as named havethe following substitutions: 6=dA thiophosphoramidite; 7=dCthiophosphoramidite; 8=dG-thiophosphoramidite; 9=dT thiophosphoramidite.

EXAMPLE 8 Further CD44 X-aptamer Substituents

In addition to ADDA, a second bead selection with a differentdrug—pteroic acid—was conducted which resulted in very similar sequencesto the ADDA-drug aptamer. See FIG. 21.

EXAMPLE 9 Further X-aptamer Substituents

It has been demonstrated by certain of the present inventors that evenconservative modifications such as introducing phosphorodithioate inplace of phosphate can have profound beneficial effects on aptamerbinding affinity. The approach of using greater chemical diversity canbe greatly expanded by using the many chemically-modified DNA or RNAreagents currently available but not compatible with SELEX. According tothe methods disclosed herein, virtually any modified nucleosidesindividually or in combination that provide favorable functional groupscan be utilized to improve molecular interactions. These includepositively-charged groups, hydrophobic groups, and amino acid sidechains. Initial selections using amino acid side chains have showntremendous promise.

Additional tested examples of chemically modified nucleotides for use inX-aptamers (X-As) include phosphorodithioates (1), 2′-O-methylphosphorodithioates (2), deoxyuridine derivatives carrying aromaticamino acid side chains such as indole (3) or phenol (4), anddeoxyuridines derivatives carrying positively charged amino acid sidechains such as guanidine (5). Certain nucleotides that are commerciallyavailable as phosphoramidites for oligonucleotide synthesis are also ofinterest, including carboxy-dT from Glen Research (6) and5-aminoallyl-dU from Berry & Assoc. (7). It has been demonstrated thatX-aptamers incorporating (1) or (2) can provide significantly enhancedbinding affinities without loss of specificity. More recently, we haveshown that nucleotides modified with certain amino acid side chains canprovide significantly enhanced aptamer binding. The above referencedmodified nucleotides (1)-(7) are shown below:

Multiple studies have demonstrated that certain amino acids areparticularly favored at the contact surfaces of interacting proteins,including antibody-antigen pairs. Mutational analyses have shown thatthese amino acids often contribute substantially to the overall energyof binding. Aromatic amino acids such as tryptophan and tyrosine appearto be especially likely to participate in such interactions, along withcertain charged amino acids including arginine and aspartate. Based onthese data, deoxyuridines modified with the indole group of tryptophan(3), the phenol group of tyrosine (4), and the guanidine group ofarginine (5) were chosen. In an initial study, aptamers to the lactatedehydrogenase protein of the malaria parasite Plasmodium falciparum(PLDH) were selected for. Selections were performed against twobead-based libraries, one consisting of standard DNA, and a second inwhich up to 10 positions within the combinatorial regions consisted of(3) the indole group of tryptophan.

No aptamers with detectable binding activity were recovered from theunmodified library. In contrast, multiple high affinity XAs wererecovered from the indole-modified library. Individual XAs wereresynthesized with appropriate indole modifications, and biolayerinterferometery was used to estimate kinetic rate constants andequilibrium affinity constants. Many bound with K_(D)<10 nM, and severalbound with estimated K_(D)<100 pM. Two of the isolated sequences areshown below with the modified deoxyuridines underlined:

PLDH-M106: SEQ ID 68 5′-GGTGTGCTGTGGCAGCGACGAATUUCAAGGGCUTTTUUUCTUGCAATCGT TCCGTGCGGGAGCCTG-3′PLDH-M86 SEQ ID 69 5′-GGTGTGCTGTGGCAGCGACGATCGGTTUCATTUCCAUUUTTUTUTCTCGT TCCGTGCGGGAGCCTG-3′

All publications, patents and patent applications cited herein arehereby incorporated by reference as if set forth in their entiretyherein. While this invention has been described with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications and combinations ofillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompasssuch modifications and enhancements.

We claim:
 1. A method for isolating a target specific X-aptamercomprising: generating a primary one bead one unique oligonucleotidesequence library by a first split and pool bead synthesis using aprogrammed synthesizer; identifying an aptamer lead sequence by targetbinding and sequence determination and pseudo-randomly insertingchemical linker modified nucleotides into the aptamer lead sequence by asecond split and pool bead synthesis; providing one or more X-ligands,wherein the X-ligands are known or predicted to bind to the target;generating a secondary one bead one unique oligonucleotide sequenceX-aptamer library using a second split and pool bead synthesis whereinthe X-aptamer library is generated by adding the one or more X-ligandsto the aptamer lead sequence and linking the one or more X-ligands viathe chemical linkers that had been pseudo-randomly inserted into theaptamer lead sequence thereby forming an X-aptamer library; andidentifying a target specific X-aptamer sequence by target binding andsequence determination.
 2. The method of claim 1, wherein the primaryone bead one unique oligonucleotide sequence library is partiallythio-modified or dithio-modified.
 3. The method of claim 1, wherein oneor more nucleotides are chemically modified in members of the primaryone bead one unique oligonucleotide sequence library.
 4. The method ofclaim 1, wherein the secondary chemical linker modified one bead oneunique oligonucleotide sequence X-aptamer library is partiallythio-modified or dithio-modified.
 5. The method of claim 1, wherein thesequence determinations are performed by generation of unmodifiedoligonucleotide versions of the target binding sequences by polymerasechain reaction (PCR) amplification and nucleic acid sequencing.
 6. Themethod of claim 1, wherein the chemical linker containing base is5-[N-(2-aminoethyl)-3-(E)-acrylamido]-2′-deoxyuridine.
 7. The method ofclaim 1 wherein the X-ligands are attached to the chemical linker priorto creation of the oligonucleotide library.
 8. The method of claim 1wherein the X-ligands are attached to the chemical linker between splitand pool steps during creation of the oligonucleotide library.
 9. Themethod of claim 1, wherein the X-ligands are coupled to the chemicallinker after generation of the X-aptamer library.
 10. The method ofclaim 1, wherein the X-ligands are added to nucleotides that incorporateX-groups by click chemistry.
 11. The method of claim 10, wherein theX-ligands are added to nucleotides that incorporate ethynyl or azideX-groups by click chemistry.
 12. The method of claim 1, wherein theX-ligands are added to nucleotides that incorporate carboxyl groups forcoupling chemistry.
 13. The method of claim 1, wherein the X-ligands areadded to nucleotides that incorporate amine groups allowing forformation of an amide bond.
 14. The method of claim 1, wherein X-ligandsare selected by one or more of: in silico screening, high-throughputchemical screening of target binding site interactions, and NMR.
 15. Amethod for isolating a target specific X-aptamer comprising: generatinga primary one bead one unique oligonucleotide sequence library by afirst split and pool bead synthesis using a programmed synthesizer;identifying an aptamer lead sequence by target binding and sequencedetermination; generating a secondary chemical linker modified one beadone unique oligonucleotide sequence X-aptamer library using a secondsplit and pool bead synthesis wherein at least one chemical linkercontaining base is pseudo-randomly inserted into the aptamer leadsequence and one or more X-ligands are linked to the at least onechemical linker thereby forming an X-ligand linked X-aptamer library;identifying a target specific X-aptamer sequence by target binding andsequence determination; and optimizing the identified target specificX-aptamer sequence by trimming of sequences that are determined to benon-target binding.
 16. A method for isolating a target specificX-aptamer comprising: generating a primary one bead one uniqueoligonucleotide sequence library by a first split and pool beadsynthesis using a programmed synthesizer; identifying an aptamer leadsequence by target binding and sequence determination; generating asecondary chemical linker modified one bead one unique oligonucleotidesequence X-aptamer library using a second split and pool bead synthesiswherein at least one chemical linker containing base is pseudo-randomlyinserted into the aptamer lead sequence and one or more X-ligands arelinked to the at least one chemical linker thereby forming an X-ligandlinked X-aptamer library; and identifying a target specific X-aptamersequence by target binding and sequence determination, wherein theX-ligand is a small molecule selected by in silico screening to bind tothe target.
 17. A method for isolating a target specific X-aptamercomprising: generating a primary one bead one unique oligonucleotidesequence library by a first split and pool bead synthesis using aprogrammed synthesizer; identifying a lead sequence by target bindingand sequence determination; generating a secondary one bead one uniqueoligonucleotide sequence X-aptamer library using a second split and poolone bead synthesis, wherein at least one or more X-ligands selected fromligands that are known or thought to bind to the target are linked tothe secondary one bead one unique oligonucleotide sequence X-aptamerlibrary using a chemical linker pseudo-randomly inserted into theaptamer lead sequence that provides for attachment of one or moreX-ligands in randomized positions into the lead sequence thereby forminga secondary X-ligand linked X-aptamer library; and identifying a targetspecific X-aptamer sequence by target binding and sequencedetermination.